The invention relates to a circuit to reduce the temperature-dependence of a predistorter diode in a predistorter for an external optical modulator. Either current or voltage impression can be used to adjust the operating point of a diode in a signal-influencing network.
U.S. Pat. No. 5,210,633, FIG. 2 shows an arrangement with current impression. In this type of arrangement the dc voltage at the diode changes as a function of the level fluctuations of the signal let through by the diode, which can be a high-frequency signal. With this familiar arrangement, the operating point for a signal amplitude value of the high-frequency signal can be optimally adjusted if the diode network is a predistorter to linearize the output signal of an optical modulator. Thus, the current source has to be tracked according to the amplitude of the input signal for an optimal linearization of the curve in this familiar arrangement.
U.S. Pat. No. 5,161,044, FIG. 15 shows a diode network to linearize the output signal of an external optical modulator, which is linearized by a light from a light source. The diodes here are connected to voltage sources. Since these are semiconductor diodes, the current-voltage curve is temperature-dependent. The operating point of the diode thus changes with no particular measure when the temperature changes. It is already known that the voltage of predistorter diodes can be tracked using a computer according to a temperature table, whereby a temperature sensor and an A/D converter as well as D/A converter are required. However, this entails a lot of work and equipment.
It is also state-of-the-art to solve the problem by keeping the predistorter diodes at a constant temperature, for example using Peltier cooling.